1. Field of the Invention
This invention concerns a coal extraction process. More especially it concerns a liquid solvent extraction process incorporating a recycle of solvent after a hydrogenation stage.
2. Description of the Related Art
It is known to extract coal using a liquid hydrogen donor solvent oil, and to hydrocrack and/or hydrogenate the coal solution over a catalyst. Such a process may be termed a two-stage direct liquefaction, as opposed to a single stage liquefaction in which hydrogen is directly supplied to a coal-solvent slurry. The two-stage process generally shows increased yields of gasoline, aviation and diesel fuel fractions per unit of hydrogen consumed, compared to the single stage process. Part of the advantage held by the two-stage process derives from the use in the extraction stage of a recycle solvent rich in hydrogen donors, which stabilise by hydrogen transfer the free radicals formed by decomposition of the coal, and hence suppress recombination reactions and solvent adduction, leading to less hydrocarbon gas formation during hydrocracking.
Our research work has shown that there is a tendency for the hydrocracking stage to result in over-hydrogenation of the solvent fraction, producing an undesired excess of naphthenes rather than hydrogen-donor hydroaromatics. This has several effects including a high consumption of expensive hydrogen and, importantly, the production of a recycle solvent containing naphthenes in such quantities that there is a tendency for high molecular weight or polar compounds to separate out of solution if the solution temperature falls. This may cause blockages in equipment.
We have discussed these problems and disclosed methods of reducing the naphthene content of recycle solvents in our published G.B. Patent No. 2,156,841, the teachings of which are incorporated herein by reference.
The recommended catalysts for hydrocracking coal solutions are molybdenum sulphide or tungsten sulphide, promoted with nickel or cobalt, on a support, which may be an alumina, alumina-silicate, silica, active carbon, magnesia, chromia, titania, carbon etc., and many suitable catalysts are commercially available as sulphur-resistant hydrotreatment or hydrocracking catalysts for petroleum refining.